For the simple reason that duct tape is sticky and will adhere to stuff. If it is wrapped around a body during decomposition, bits of skin and facial hair (stuff that is more resistant to decomposition) would have been attached to it and traces would still be there when it was recovered. They didn't find anything like that, where ever the duct tape was originally, it probably wasn't on her face.
Remember the hair with the "death ring" or whatever they called it? That doesn't happen right away, you have to have decomposition first. And if the body was contained in a bag, that hair could not have gotten out. In other words, if it really was a hair from her corpse, the corpse must have been somewhere outside the bag for a considerably length of time. That could not have been in the car itself because then body fluids would have been found, and they were not. So where the hell was it during that time?
So, for the hair to be in the car, and assuming the car was used to transport the wrapped body to the swamp, the body must have been lying in the house and decomposing for a long time. Remember, all the stuff used to contain the body apparently came from the house. The hair could have been then transferred to the outside of the bag during wrapping and subsequently deposited in the car. Note: the body must have been in the house for a considerable period of time for that to have happened. How can the parents have been unaware of that when they lived there? How could Casey have done that when her parents said she took off right after the last time they saw Caylee alive? The parents version of events is not credible, and they are the primary line of evidence implicating Casey.
The prosecutions own evidence proves that their theory of events could not possibly have happened the way they said.